1. Field of the Invention
The present invention relates to an electrode for a capacitor having an electrode charge eliminator and an electrode body which is contacted with the electrode charge eliminator and on whose surface a dielectric layer is arranged. The present invention further relates to a capacitor having such an electrode.
2. Description of the Prior Art
Electrodes for capacitors are known, wherein the electrode body is a porous sintered compact composed of niobium and wherein the dielectric layer is a layer composed of niobium Nb2O5 produced by anodic oxidization of the porous sintered compact. Basically, such electrodes can be used as anodes for electrolyte capacitors wherein the pores of the sintered compact are filled with a cathode material, which is contacted with a cathode charge eliminator.
The known electrodes have the disadvantage that the dielectric layer does not have a sufficient long-time stability, since the oxygen can diffuse from the dielectric layer into the electrode body. Suboxides such as the semiconducting NbO2 or, respectively, the metallically conducting NbO are formed in the dielectric as a result of the oxygen output. This makes the dielectric effectively thinner so that electric strength is lost and the capacitor fails.
Therefore, the present invention is directed to providing an electrode for capacitors which exhibits a high long-time stability.
Accordingly, the present invention proposes an electrode for a capacitor, with an electrode charge eliminator and with an electrode body which is contacted with the electrode charge eliminator, and on whose surface a dielectric layer is arranged. An intermediate layer blocking the exchange of matter between the dielectric layer and the electrode body is arranged between the electrode body and the dielectric layer.
Moreover, the present invention proposes a capacitor with an inventive electrode, wherein the capacitor includes a counter electrode such that the dielectric layer is arranged between the intermediate layer and the counter electrode.
Furthermore, the present invention proposes a capacitor, wherein the surfaces of the pores of the electrode are covered with a counter electrode and wherein the counter electrode is contacted with a counter electrode charge eliminator.
The inventive electrode has the advantage that material changes of the dielectric layer or of the electrode body can be effectively blocked due to the intermediate layer. As a result, the stability of the electrode body or, respectively, of the dielectric layer improves, and the electrode, or the capacitor produced by it, exhibits an improved long-time stability.
The present invention can be advantageously utilized for an electrode whose electrode body is a porous body. Porous bodies are characterized by a large surface, so that capacitors having large capacities can be created when the surfaces of the pores, corresponding to the inventive capacitor, are covered with a counter electrode.
The porous electrode body can be produced by sintering a powder or a paste, for example. The sintering of a green compact of powder containing tantalum or niobium or, respectively, of a correspondingly suitable paste is particularly considered.
The dielectric layer of the electrode can be produced, for example, by oxidizing a surface layer of a body whose non-oxidized residue forms the electrode body. In this case, it is particularly advantageous when the dielectric layer is selected such that it impairs the diffusion of oxygen. As a result, the diffusion of oxygen from the dielectric layer into the electrode body can be blocked, so that the change of the dielectric layer, which progresses in the course of time due to the loss of oxygen, can be effectively reduced.
Such an intermediate layer, which blocks the diffusion of oxygen, can be an intermediate layer containing scandium, yttrium, a lanthanide, titan, zircon, vanadium, chrome or molybdenum. Lanthanide, lanthanum, cerium, praseodymium, neodymium, polonium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, thulium or lutetium also can be taken into consideration. As a result of the low diffusion rate of oxygen, all these materials are well suited as a diffusion barrier for oxygen.
In the case of the electrode body including niobium and the dielectric layer including Nb2O5, it is particularly advantageous to utilize an intermediate layer that includes vanadium, as experiments have shown.
The thickness of the intermediate layer should thereby have at least two atomic monolayers. Thicker intermediate layers, however, are also conceivable.
The inventive electrode can be advantageously produced from a material including a component A and a component B, wherein the surface energy of the component B is less than the surface energy of the component A, and wherein the intermediate layer includes a part of the component B segregated from the starting material. The component B thereby can be present in the form of a dopant of the material.
Since the intermediate layer is produced by segregation of a component B, its production is particularly simple and occurs without further outlay, since the intermediate layer can be automatically produced at the inventively appropriate location; namely, at the surface of the electrode body. As a material, a metallic alloy, wherein the portion of the component B typically is between 10 and 50 weight per centxe2x80x94ppm, can be particularly considered.
A metallic alloy has the advantage that the electrode body produced by it simultaneously exhibits the electrically conducing properties required for a capacitor. It is particularly advantageous when the component A of the alloy is a valve metal that is suitable for forming a dielectric or when the component A is an alloy containing such a valve metal. Valve metals exhibit the property of being capable of forming a suitable dielectric layer by oxidization. For example, tantalum or niobium can be used as valve metal. As a valve-metalliferous alloy, a tantalum/niobium alloy, for example, can be used which also forms a corresponding mixed oxide exhibiting advantageous properties when the dielectric layer is formed by oxidization. In these cases, the component B can be respectively one of the aforementioned individual metals, for example vanadium, or can be a combination thereof.
Furthermore, it is advantageous when the electrode body is produced from an alloy containing niobium or vanadium, since an intermediate layer composed of segregated vanadium can be simply formed due to the different surface energies of niobium (xcex3=2.983 Jmxe2x88x922) and vanadium (xcex3=2.876 Jmxe2x88x922). The segregation of the vanadium to the surface of the niobium, for example, can occur via a correspondingly suitable heat treatment, subsequent to the oxidization of the electrode body.
Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawings.